Focus detecting device for removing vignetting effects

ABSTRACT

A focus detecting device for detecting the focus adjusted state of an imaging optical system with respect to an object body comprises first and second photoelectric element arrays each including a plurality of photoelectric elements arranged in one direction, a focus detecting optical system for forming first and second optical images of the body on or near the first and second arrays by first and second light beams passed through different first and second areas, respectively, of the exit pupil of the imaging optical system, the first array producing a series of first output signals having a distribution pattern associated with the illumination intensity distribution pattern of the first image, the second array producing a series of second output signals having a distribution pattern associated with the illumination intensity distribution pattern of the second image, focus detection means for producing a focus detection signal representative of the focus adjusted state of the imaging optical system, on the basis of the first and second output signals, and vignetting detection means for detecting, on the basis of the first and second output signals, the vignetting influence of the first and second output signals by the vignetting of the first and second images caused by the imaging optical system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a focus detecting device for forming opticalimages of substantially the same object on a pair of photoelectricelement arrays by two light beams passed through two different areas ofthe exit pupil of an imaging optical system such as a photo-taking lensand effecting focus detection from the relative image displacementamount of the two optical images.

2. Description of the Prior Art

Focus detecting devices of this kind have been adopted in single lensreflex cameras, video cameras, etc., and generally the following twotypes of such devices are known. The first type is a system in which, asshown in U.S. Pat. No. 4,264,810, primary images of an object formed bya photo-taking lens are re-imaged on a pair of photoelectric elementarrays by a pair of re-imaging optical system, and the second type is asystem in which, as described in U.S. Pat. No. 4,185,191, a row ofminute lenses are disposed substantially on the focal plane of aphoto-taking lens and a pair of photoelectric elements are providedbehind each of the minute lenses.

Such focus detecting devices suffer from a disadvantage that if a pairof light beams used for focus detection, i.e., light beams formingoptical images on the pair of photoelectric element arrays, arevignetted by the aperture or the like of the photo-taking lens, thefocus detecting accuracy will be reduced greatly.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a focusdetecting device in which the reduction in focus detection accuracy canbe greatly lessened even if the focus detecting light beams arevignetted by the aperture or the like of the imaging optical system.

The invention will become fully apparent from the following detaileddescription thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a focus detecting optical systemaccording to an embodiment of the present invention.

FIG. 2 is a graph showing the relation between the fully open aperturevalue Fo of the photo-taking lens and the defocus amount conversionfactor 1/θ.

FIGS. 3A, 3B and 3C are plan views schematically showing the positionsof the optical images on photoelectric element arrays.

FIGS. 4A and 4B are graphs showing the relative position displacementamount of the optical images on a pair of photoelectric element arrayswhich corresponds to the location.

FIG. 5 is a block diagram showing a signal processing system accordingto an embodiment of the present invention.

FIGS. 6A, 6B and 6C are graphs showing the image output of thephotoelectric element array and the manner in which this image output isdivided into a plurality of areas.

FIG. 7 is a flow chart showing part of the operation of an embodiment.

FIGS. 8A, 8B and 9 are block diagrams showing the specific constructionsof image displacement amount correction means.

FIGS. 10A and 10B are graphs showing the image output and the sharpnessof the variation in the image output, respectively.

FIG. 11 is a block diagram showing an improved example of the aboveembodiment.

FIG. 12 is a graph showing the relation between the position of the bestimaging plane and the aperture value of the photo-taking lens.

FIG. 13A shows another focus detecting optical system.

FIG. 13B is a front view of the photoelectric element array of FIG. 13A.

FIGS. 14A and 14B show the positional relation between the focusdetecting light beam when subjected to the influence of vignetting andthe photoelectric element array.

FIGS. 15A, 15B and 15C are graphs showing the vignetting characteristic,the brightness distribution of an object to be photographed, and thephotoelectric output pattern when subjected to the influence ofvignetting, respectively.

FIG. 16 is a block diagram showing another embodiment of the presentinvention.

FIGS. 17A, 17B and 17C are block diagrams showing specific examples ofthe construction of the vignetting detection means and mode detectionmeans of FIG. 16.

FIG. 18 is a block diagram showing still another embodiment of thepresent invention.

FIGS. 19A and 19B are graphs showing the characteristics of the filtermeans of FIG. 18.

FIG. 20 is a block diagram showing yet still another embodiment of thepresent invention.

FIG. 21 is a block diagram showing a specific example of theconstruction of the second difference means of FIG. 20.

FIG. 22 is a graph showing the logarithmic conversion characteristic.

FIGS. 23A and 23B are circuit diagrams showing a logarithmic conversioncircuit.

FIG. 24 is a block diagram showing a modification of the logarithmicmeans of FIG. 20.

FIG. 25 is a graph showing the logarithmic conversion characteristic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a field lens 2 is provided on or near thepredetermined focal plane of an interchangeable photo-taking lens 1 suchas the photo-taking lens of a single lens reflex camera. This field lens2 is formed on one end of a focus detecting optical block 3 formed oftransparent plastic or glass, and the other portion thereof than therectangular focus detecting area 4 is treated for light interception.The formation of this focus detecting area 4 may be such that alight-intercepting plate having the rectangular opening is disposedimmediately in front of the field lens 2. The light beams 101A and 101Bfrom an object to be photographed passed through different areas 100Aand 100B of the exit pupil of the photo-taking lens 1 form the primaryimages of the object to be photographed near the detecting area 4. Thelight beams 101A and 101B having entered the focus detecting opticalsystem 3 from the detecting area 4 are both reflected upwardly by areflecting surface 5, and one light beam 101A is further reflected by aconcave mirror 6A which serves as a re-imaging optical system, and formsthe secondary image of the object to be photographed on a photoelectricelement carry 7A. The other light beam 101B is further reflected by asimilar concave mirror 6B and forms the secondary image of the object tobe photographed on a photoelectric element array 7B. Each of the arrays7A and 7B is comprised of a number of photoelectric elements arranged inone direction. Of course, the juxtaposed concave mirrors 6A and 6B areinclined at different angles to form secondary images on the juxtaposedphotoelectric element arrays 7A and 7B. The power of the field lens 2 isdetermined so that the concave mirrors 6A, 6B and hatching areas 100A,100B are in conjugate relation with each other. Accordingly, the sizesof the hatching areas 100A and 100B are determined by the sizes,respectively, of the corresponding concave mirrors 6A and 6B. Only thelight beams 101A and 101B passed through the hatching areas 100A and100B of the photo-taking lens 1 arrive at the photoelectric elementarrays 7A and 7B and therefore, these areas 100A and 100B are referredto as the set pupils. In this example shown, the exit pupil position ofthe photo-taking lens and the setting pupil position of the focusdetecting optical system 3 are coincident with each other in thedirection of the optic axis. Also, the pair of concave mirrors 6A and 6Bare adjusted so that the images of the photoelectric element arrays 7Aand 7B overlap each other on the detecting area 4.

The relation between the amount of relative image displacement of theoptical images on the photoelectric element arrays 7A, 7B and the amountof defocus (the defocus amount is the amount of displacement between thepredetermined focal plane of the photo-taking lens and an object imageformed by the photo-taking lens along the direction of the optical axis)will hereinafter be described.

When the amount of defocus is P and the amount of relative imagedisplacement is ZT, there is the following relation: ##EQU1## where K isa coefficient determined by the characteristic of the focus detectingoptical system 3 including the magnifications of the concave mirrors 6Aand 6B, and θ is the angle formed by straight lines passing through thecenter of the detecting area 4 and the centers of the areas of the setpupils 100A and 100B, and will hereinafter be referred to as thedetection opening angle.

The amount of relative image displacement ZT is a value found from theoperation of the image outputs of the photoelectric element arrays 7Aand 7B, and K is a value determined only by the focus detecting opticalsystem as described above. However, the detection opening angle θ is avalue varied by the brightness of the photo-taking lens, i.e., the fullyopen aperture value Fo. More particularly, when the diameter of the exitpupil of the photo-taking lens determined by the fully open aperturevalue Fo, of the photo-taking lens 1, is greater than that of thecircumscribed circle of the set pupils 100A, 100B determined by thesizes of the concave mirrors 6A, 6B, the light beams passing through theareas 100A and 100B are not vignetted at all and the then detectionopening angle θ is θ₁ as shown. However, if a lens having a greaterfully open aperture value, is used as the photo-taking lens 1 and thediameter of the exit pupil thereof is smaller as indicated by dottedline 103 or 104, said light beams will be vignetted and only the lightbeams passing through the set pupils 100A and 100B within the dottedline 103 or 104 will arrive at the photoelectric element arrays 7A and7B after all. Accordingly, the then detection opening angle is smallerthan said value θ₁.

FIG. 2 is a graph showing the relation between the fully open aperturevalue Fo of the photo-taking lens and the conversion factor 1/θ. In thisgraph, the pupil diameters 102, 103 and 104 are shown as F1=2.4, F2=4and F3=5.6, respectively. When the open aperture value of thephoto-taking lens is smaller than F1, said vignetting does not occur andtherefore the factor 1/θ is constant, but when the fully open aperturevalue of the photo-taking lens is greater than F1, the amount ofvignetting becomes greater in accordance with the increase in theaperture value and therefore the factor 1/θ also increases.

Accordingly, when the amount of image displacement ZT is to be convertedinto the amount of defocus P, the detection opening angle θ must becorrected in accordance with the degree to which the light beam passingthrough the set pupils 100A and 100B are vignetted by the photo-takinglens, and more specifically, in accordance with the fully open aperturevalue of the photo-taking lens.

Of course, in the foregoing description, the abscissa of FIG. 2 is theopen aperture value Fo because, in the photo-taking lens of a singlelens reflex camera, focus detection is generally effected in the fullyopen aperture state, but where focus detection is effected in a state inwhich the aperture has been stopped down from the open aperture, theamounts of vignetting of the focus detecting light beams 101A and 101Bin such stopped-down state offer a problem and therefore, the value ofthe abscissa of FIG. 2 is the aperture value in such stopped-down state.Generally speaking, the degree of vignetting is determined by theaperture value of the photo-taking lens during focus detection andtherefore, the abscissa of FIG. 2 represents the aperture value of thephoto-taking lens during focus detection.

Increasing the dimensions of the setting pupils 100A and 100B leads toan advantage that the quantity of light entering the photoelectricelement arrays 7A, 7B naturally increases and the S/N ratio of the imageoutputs of the arrays 7A and 7B is improved. When the set pupils 100Aand 100B are to be determined greatly, it is desirable to determine theshape of the areas 100A and 100B so that, as shown in FIG. 1, thedimensions in the direction of arrangement thereof (the lateraldirection as viewed in FIG. 1) are greater than the dimensions in adirection perpendicular thereto. The reason is that if the dimensions inthe perpendicular direction is made greater, the angle of relativeinclination of the concave mirrors 6A and 6B will become greater andthis will aggravate the aberrations of the focus detecting opticalsystem 3.

An influence which the vignetting of the focus detecting light beamscaused by the aperture diameter of the photo-taking lens imparts to theconversion factor of the amount of defocus has been described above, andanother influence which said vignetting imparts to the amount of defocuswill hereinafter be described.

It is necessary as a matter of course that the amount of relative imagedisplacement of the optical image on the photoelectric element array 7Aand the optical image on the photoelectric element array 7B be equal atany positions on the photoelectric element arrays. However, ifvignetting occurs to the focus detecting light beams, the amount ofrelative image displacement will differ depending on the positions onthe photoelectric element arrays.

FIGS. 3A, 3B and 3C show the images of three point light sources lyingequidistantly from the camera, on the photoelectric element arrays 7Aand 7B. FIG. 3A shows the images X_(0a), X_(1a), X_(2a), X_(0b), X_(1b)and X_(2b) of three point light sources when a photo-taking lens of thefully open aperture value F2, i.e., exit pupil diameter 103, is used,and the images X_(0a) and X_(0b) at the center of the photoelectricelement arrays are point images, while the images X_(1a), X_(1b), X_(2a)and X_(2b) near the opposite ends of the photoelectric element arraysare blurred images because of the aberrations of the focus detectingoptical system 3. FIGS. 3B and 3C show the images when photo-takinglenses of the fully open aperture value F1 and F3, respectively, areused. The focus detecting optical system 3 is pre-adjusted so that whena photo-taking lens of the aperture value F2 causing vignetting is used,the images X_(0a) and X_(0b) at the center of the photoelectric elementarrays are substantially point images as shown in FIG. 3A and therelative position of the centers of gravity of the images is coincidentwith respect to the corresponding images X_(0a) and X_(0b), X_(1a) andX_(1b), and X_(2a) and X_(2b). If the focus detecting optical system 3is so adjusted, when a photo-taking lens having aperture value smallerthan F2, for example, a photo-taking lens of the aperture value F1 isused, the degree of vignetting of the focus detecting light beams issmaller than in the case of a photo-taking lens of aperture value F2(there is no vignetting for F1) and therefore, as shown in FIG. 3B, theblurred images X_(1a), X_(1b), X_(2a) and X_(2b) become larger and thecenter of gravity of the corresponding images is displaced in theopposite direction and accordingly, the relative position of thecorresponding images X_(1a) and X_(1b), X_(2a) and X_(2b) is displacedby an amount indicated by arrow. On the other hand, when a photo-takinglens of F3 greater than F2 is used, the degree of said vignetting isgreater in the photo-taking lens of F3 than in the photo-taking lens ofF2 and therefore, as shown in FIG. 3C, the relative position of thecorresponding images near the end portions of the arrays 7A and 7B isdisplaced conversely to the case of FIG. 3B.

Such displacement of the relative position of the optical imagescorresponding to the location on the photoelectric element arraysnaturally reduces the detection accuracy of the amount of defocus andmust be corrected.

FIG. 4A is a graph in which the abscissa represents the position x onthe photoelectric element array and the ordinate represents the amountof relative position displacement of said optical image and which showsthe variation in the amount of relative position displacement when theaperture value of the photo-taking lens varies. A function T(F2, x)relates to the case where the photo-taking lens of aperture value F2shown in FIG. 3A is used, and in this case, no relative positiondisplacement occurs. Functions T(F1, x) and T(F3, x) show the caseswhere the photo-taking lenses of aperture values F1 and F3 shown inFIGS. 3B and 3C are used, and in these cases, as x becomes greater, theamount of position displacement becomes greater.

In the foregoing description, the focus detecting optical system hasbeen described as being pre-adjusted so that said relative positiondisplacement never occurs with regard to the images at the center of thephotoelectric element arrays and that said relative positiondisplacement does not occur over the entire area of the photoelectricelement arrays when the photo-taking lens of medium aperture value F2 isused, but such adjustment is not limited to what has been describedabove.

Description will hereinafter be made of an embodiment of the presentinvention in which both of the two factors of the defocus amountdetection accuracy by the above-described vignetting of the focusdetecting light beams are corrected. This embodiment is an example inwhich the focus detecting device according to the present invention isincorporated into a single lens reflex camera.

Referring to FIG. 5, correction data memory means 10 includes acorrection data memory 11 for amount of image displacement and a defocusamount conversion factor memory 12. The memory 11 stores therein therelative position displacement function T (F, x) as shown in FIG. 4Awhich has been premeasured for each photo-taking lens used, inconnection with the fully open aperture value F of the photo-takinglens. The memory 12 stores therein the defocus amount conversion factor1/θ as shown in FIG. 2, in connection with the fully open aperture valueF of the photo-taking lens.

A pair of photoelectric element arrays 7A and 7B each comprising animage sensor such as CCD are juxtaposed in a photoelectric convertingunit 13. The image of the same object to be photographed is formed oneach photoelectric element array by the focus detecting optical systemof FIG. 1. The image output a1 . . . aN from the array 7A istime-serially A/D-converted by an A/D converter 14 and stored in datamemory means 16 within a microcomputer 15, and likewise, the imageoutput b1 . . . bN from the array 7B is stored in the data memory means16 through the A/D converter 14. Image displacement operation means 17calculates the amount of relative displacement of the two image outputs,i.e., the amount of relative displacement of the optical images on thepair of arrays, on the basis of said pair of image outputs a1 . . . aNand b1 . . . bN. Of course, the data used for the image displacementoperation need not always be the direct image outputs of the arrays 7Aand 7B, but may be image outputs obtained by suitably filtering orsampling these outputs.

Image displacement correction means 18 corrects the output of the imagedisplacement operation means 17 by the content of the correction datamemory 11 for amount of image displacement in accordance with the fullyopen aperture value of the photo-taking lens used. Defocus amountconversion means 19 converts the output of the image displacementcorrection means 18 into a defocus amount by the content of the defocusamount conversion factor memory 12 in accordance with said aperturevalue. The fully open aperture value of the photo-taking lens used isautomatically or manually input to the fully open aperture value inputmeans 20 in response to the mounting operation of the photo-taking lens.Display and driving means 21 displays the focus adjusted state on thebasis of the defocus amount and drives the photo-taking lens to itsin-focus position.

The operation will be described hereinafter.

The image outputs a1 . . . aN and b1 . . . bN from the pair of arrays 7Aand 7B are A/D-converted and then stored in the data memory means 16.The image output a1 . . . aN stored in the data memory means 16 isillustrated in FIG. 6A. The image displacement operation means 17divides this image output, for example, into five areas X-2, X-1, X0, X1and X2 as shown in FIG. 6B or 6C, and also divides the image output b1 .. . bN into five areas X-2, X-1, X0, X1 and X2, and individuallyoperates the partial image displacement amount Z(xi) relating to thecenter xi of the partial area Xi, from the image output of each partialarea Xi. The dividing method of FIG. 6B is an example in which thedivided image outputs do not overlap one another, and the dividingmethod of Figure 6C is an example in which the divided image outputspartly overlap one another. The dividing method of FIG. 6C, as comparedwith that of FIG. 6B, has an advantage that the number of the data ineach divided image output (the data correspond, for example, to theoutput of the photoelectric element) can be increased. The imagedisplacement correction means 18 reads out, from the correction datamemory 11, the correction data T(Fa, x) corresponding to the fully openaperture value information Fa of the photo-taking lens used which hasbeen input to the fully open aperture value input means 20, andcalculates the partial correction amount T(Fa, xi) of the location xifrom this correction data. Thereafter, the correction means 18 subtractsthe partial correction amount T(Fa, xi) from the partial imagedisplacement amount Z(xi) and obtains the corrected partial imagedisplacement amount ZT(xi). That is, it obtains ZT(xi)=Z(xi) -T(Fa, xi).

In this manner, the correction means 18 obtains the corrected partialimage displacement amount ZT(xi) relating to each partial area Xi andfor example, calculates the simple average ΣZT(xi)/5 of these values asthe final image displacement amount ZT. Various methods of obtaining thefinal image displacement amount ZT from the partial image displacementamount ZT(xi) relating to each partial area Xi would occur to mind inaccordance with the purpose thereof, and some examples of those methodswill be shown below.

(A1) As described above, the average value of the corrected partialimage displacement amounts ZT(xi) is regarded as the final imagedisplacement amount ZT.

(A2) The average value of the remaining corrected partial imagedisplacement amounts except the maximum and the minimum of the correctedpartial image displacement amounts ZT(xi) is regarded as the final imagedisplacement amount ZT.

(A3) The middle one of the corrected partial image displacement amountsZT(xi) when arranged in the order from the greater one is regarded asthe final image displacement amount ZT.

(A4) The corrected partial image displacement amount ZT(xi) for thepartial area (xi) in which the information amount E(x) to be describedbecomes the maximum is regarded as the final image displacement amountZT.

(A5) The average value of the corrected partial image displacementamounts ZT(xi) for a plurality of partial area Xi in which saidinformation amounts are relatively large, or the average value obtainedby multiplying the corrected partial image displacement amounts ZT(xi)by the respective coefficients having magnitudes according to saidinformation amounts and by adding the products (multiplied values) isregarded as the final image displacement amount ZT.

The defocus amount conversion means 19 reads out, from the conversionfactor memory 12, the conversion factor 1/θ corresponding to the fullyopen aperture value information Fa from the fully open aperture valueinput means 20, and converts the final image displacement amount ZT ofthe correction means 18 into a dofocus amount P by the use of thisconversion factor 1/θ and another conversion factor K determined by thecharacteristic of the focus detecting optical system. That is, ##EQU2##The display and driving means 21 is operated on the basis of thisdefocus amount P.

In order to effect the algorithm for calculating the partial imagedisplacement operation from the data of each partial area Xi, it ispossible to use, for example, means for Fourier-converting the imageoutput and comparing the phase (U.S. Pat. No. 4,264,810) or means foreffecting correlation operation and obtaining a shift amount whichprovides a maximum correlation (U.S. Pat. No. 4,333,007), and further,it is also possible to use the correlation operation method disclosed inU.S. application Ser. No. 575,154 by the same assignee as the assigneeof the present invention. In this last cited correlation operationmethod, the correlation amount C(L) when a pair of data rows A1 . . . ANand B1 . . . BN have been relatively shifted by a predetermined amount Lis obtained from ##EQU3## and when C(L)≦C(L-1) and C(L)<C(L+1) aresatisfied, the following values DL, E, Cext and Lm are obtained:

    DL={C(L-1)-C(L+1)}×1/2                               (2)

    E=Max{C(L+1)-C(L), C(L-1)-C(L)}                            (3)

where Max{Ca, Cb} means selecting the greater one of Ca and Cb.

    Cext=C(L)-|DL|                           (4')

    Cext={C(L)-|DL|}/E                       (4)

    Lm=L+DL/E                                                  (5)

When the range of the data Ai and Bi, used for calculating thecorrelation amount C(L) is operated being regarded as said each partialarea Xi, said Lm becomes the partial image displacement amount Z(xi).When the number of the photoelectric elements included in the partialarea Xi is small, the use of said Fourier conversion method would resultin higher accuracy.

Where a deep object to be photographed is imaged on the array, if theimage displacement amount is calculated by the use of the image outputof the entire area of the array, to which portion of the object of largedepth the photo-taking lens is automatically focused will be entirelyunclear.

The problem regarding such a deep object to be photographed can besolved as follows by operating the partial image displacement amountZ(xi) for each partial area Xi:

(B1) If the smallest image displacement amount is selected from among aplurality of partial image displacement amounts Z(xi) and the finalimage displacement amount ZT is obtained on the basis thereof, thedefocus amount regarding the nearest portion of the deep object to bephotographed can be obtained and conversely, by the selection of thegreatest partial image displacement amount, the defocus amount regardingthe distant portion can be obtained, and further, by the selection ofthe medium partial image displacement amount, the defocus amountregarding the portion of medium distance can be obtained.

(B2) If some of a plurality of partial image displacement amounts Z(xi)assume a substantially equal value, if that value is selected and thefinal image displacement amount ZT is obtained on the basis thereof,there can be obtained the defocus amount regarding an object to bephotographed which occupies a relatively wide area.

(B3) If the partial image displacement amount in the partial area Xi inwhich the information amount E(xi) to be described is greatest isselected and the final image displacement amount ZT is obtained on thebasis thereof, there can be obtained the defocus amount regarding anobject to be photographed having the most information for focusdetection, generally, an object to be photographed having the bestcontrast.

A specific example of the case where the partial image displacementamount is calculated as described above will now be described by the useof a flow chart.

In FIG. 7, at step ○ , the partial image displacement amount Z(xi) andinformation amount E(xi) relating to each partial area Xi are calculatedby the image displacement operation means 17. The information amountE(xi) represents the degree of reliability of the corresponding partialimage displacement amount Z(xi) and the greater is the value of thisinformation amount, the higher is the accuracy of the correspondingpartial image displacement amount. More specifically, if imagedisplacement operation is effected by the phase comparison after Fourierconversion, the value related to the amplitude after Fourier conversion(for example, the information amounts r1, r1', r2 and r2' shown in U.S.Pat. No. 4,336,450 correspond thereto) can be used as the informationamount, and where the image displacement operation is the correlationmethod described in the aforementioned U.S. Pat. No. 4,333,007 or U.S.application Ser. No. 575,154, a self-correlated value Wm to be describedcan be used, and particularly in the case of U.S. Pat. No. 4,333,007,the value Dm shown therein can be used, and where the image displacementoperation is the correlation operation of U.S. application Ser. No.575,154, E of the aforementioned equation (3) can also be used. At step○ , the information amount E(xi) of each partial area Xi is comparedwith a predetermined threshold value Eth and a partial area Xj having aninformation amount E(xj) of a value greater than this threshold value isselected. At step ○ , a correction data T(Fa, x) corresponding to theaperture value information Fa input to the fully open aperture valueinput means 20 is read out from the correction data memory 11, and thepartial correction amount T(Fa, xj) for the selected partial area Xj iscalculated therefrom and the partial image displacement amount Z(xj) forthe selected partial area Xj is selected from among the partial imagedisplacement amounts Z(xi). At step ○ , the corrected partial imagedisplacement amount ZT(xj) relating to the selected area Xj iscalculated from ZT(xj)=Z(xj)-T(Fa, xj). At step ○ , whether thefluctuation of the corrected partial image displacement amounts ZT(xj)obtained at step ○4 is smaller than a predetermined value ΔZ, and morespecifically, whether the difference between the maximum value and theminimum value of the corrected partial image displacement amounts ZT(xj)is smaller than a predetermined value ΔZ is discriminated and, when itis smaller than the predetermined value, the object to be photographedis judged as having no depth and the program shifts to step ○6 , andwhen it is not smaller than the predetermined value, the object to bephotographed is judged as being deep and the program shifts to step ○7 .At step ○6 , the final image displacement amount ZT is calculated, forexample, by any one of the aforementioned processes (A1) to (A5). Atstep ○7 , the final image displacement amount ZT is calculated, forexample, by any one of the afore-mentioned processes (B1) to (B3).

FIG. 8A shows a specific example of the operation of the imagedisplacement correction means 18 of FIG. 5 in a detailed block diagram.The image displacement amounts Z(x₁), Z(x₂), Z(x₃), Z(x₄) and Z(x₅) andthe information amounts E(x₁), E(x₂), E(x₃), E(x₄) and E(x₅) relating tofive areas Xi operated by the image displacement operation means 17 arestored in a memory 18A. Comparing means 18B compares each of theinformation amounts E(x₁)-E(x₅) with the predetermined value Eth in amemory 18C and causes a memory 18D to store therein the partial areas Xjrelating to the information amount greater than said predetermined valueand the number M of such areas. Correction means 18E receives as inputthe selected partial areas Xj from a memory 18D, whereby it calculatesthe partial correction amount T(Fa, xj) from the correction data T(Fa,x) and calculates the corrected partial image displacement amountsZT(xj) relating to the selected partial areas Xj. Statistics processingmeans 18F effects statistics processes (A1), (A2) or (A5) from theseimage displacement amounts ZT(xj), the number M and the informationamount E(xj) and obtains the final image displacement amount ZT.

FIG. 8B shows an example in which the aforementioned process (A3) iseffected. Maximum value detecting means 18G detects the maximum value ofthe information amounts E(x₁)-E(x₅) and causes a memory 18H to storetherein a partial area Xq which provides this maximum value. Correctionmeans 18E corrects the partial image displacement amount Z(xq) regardingthis area Xq.

FIG. 9 is a block diagram of the steps ○5 , ○6 and ○7 of FIG. 7. In FIG.9, blocks 18A-18E are entirely identical to those of FIG. 8A.Subtraction means 18I finds the maximum and the minimum of the correctedpartial image displacement amount ZT(xj), and then calculates thedifference ΔZT between the two. Comparing means 18J compares thisdifference ΔZT with a predetermined value ΔZ in a memory 18K. Statisticsprocessing means 18L judges the object to be photographed as having nodepth when ΔZT<ΔZ and calculates the final image displacement amount ZTby the process (A1), and judges the object to be photographed as beingdeep when ΔZT≧ΔZ and regards as the final image displacement amount thecorrected image displacement amount ZT(xq) corresponding to the maximumE(xq) of the information amounts E(x₁)-E(x₅), for example.

The above-described embodiment is an example in which a plurality ofpartial image displacement amounts are calculated from a pair of imageoutputs, and a second embodiment in which a single image displacementamount is calculated will now be described.

FIG. 10A shows one of a pair of data trains A1 . . . AN and B1 . . . BNstored in the data memory means 16 of FIG. 5. The data trains may be theimage outputs of the photoelectric element arrays or the image outputssubjected to filtering or sampling, as previously mentioned.

In FIG. 5, the image displacement operation means 17 uses a pair of datatrains A1 . . . AN and B1 . . . BN stored in the data memory means 16and obtains the correlation amount C(L) for each shift amount L whichshifting one data train A1 . . . AN by predetermined amounts L relativeto the other data train B1 . . . BN. ##EQU4##

The shift amount Lm for which this function C(L) is smallest is obtainedas the image displacement amount. The image displacement amount Lmobtained by the correlation operation includes an error attributable tothe position displacement by the vignetting as described above andtherefore, this image displacement amount Lm must be corrected by thecorrection data T(Fa, x) relating to the aperture value of thephoto-taking lens used. However, this image displacement amount iscalculated from the entire area of the data trains A1 . . . AN and B1 .. . BN and therefore, a problem arises as to which area of thecorrection data T(Fa, x) as the correction amount.

This problem is solved in the following manner.

Not all portions of the data trains A1 . . . AN and B1 . . . BN equallycontribute to said correlation amount C(L), but as shown in FIG. 10A, aportion Y in which a sharp variation in the data train occurscontributes greatly and portions in which gentle variation occurscontributes less. Accordingly, the degree of said contribution for eachlocation x of the data trains A1 . . . AN and B1 . . . BN (hereinafterreferred to as the degree of contribution) may be obtained and thecorrection amount may be obtained from the degree of contributioncorresponding to the location and the correction data T(Fa, x). Thisdegree of contribution Wm can be calculated, for example, from thedifference between adjacent data of the data trains.

That is, Wm=|Am-Am+1| or |Bm-Bm+1|. This value Wm is shown in FIG. 10B.

Of course, |Am-Am+1|+|Bm-Bm+1| can also be used as Wm.

The correction amount ST is

    ST=ΣSm·Wm/ΣWm,

where Sm is T(Fa, x) when x=m. Accordingly, the corrected imagedisplacement amount ZT can be found from the following equation:

    ZT=Lm-ST

Also, the final image displacement amount ZT can be determined asfollows. Said Wm is calculated from the image output A1 . . . AN or B1 .. . BN stored in the data memory means 16, and data AP or BP whichprovides the maximum of Wm over the entire area of the image output isobtained. When the position of this data AP or BP on the coordinatesaxis x of FIG. 6B is xp, a predetermined range area Xp about theposition xp is determined and a partial image displacement amount Z(xp)is calculated relative to the image output in this area. This iscorrected by the correction amount T(Fa, xp) at said position andregarded as the final image displacement amount ZT.

That is, ZT=Z(xp)-T(Fa, xp).

FIG. 11 shows an improved example of the FIG. 5 arrangement. Withincorrection data memory means 10, in addition to a correction data memory11 for amount of image displacement and a defocus amount conversionfactor memory 12, there are provided a data memory 22 for best imageplane displacement, a data memory 23 for adjustment error correction, adata memory 24 for error correction, a data memory 25 for first offsetand a data memory 26 for second offset. The data memory 22 for bestimage plane displacement serves to store therein a function β(F) shownin FIG. 12, and this function β(F) represents the amount of displacementof the best imaging plane of the object to be photographed in thedirection of the optic axis when the aperture of the photo-taking lensis stopped down. The data memory 23 for adjustment error correctionstores therein the remaining superposition error is a function U(x)after the adjustment of superposing on the focus detecting area 4 theimages of the photoelectric element arrays 7A and 7B by the concavemirrors 6A and 6B of FIG. 1.

Also, the data memory 24 for error correction stores therein a functionδ(F) shown in FIG. 4B. The correction data memory 11 for amount of imagedisplacement stores therein a function T(F, x) shown in FIG. 4A inconnection with the aperture value of the photo-taking lens on thepremise that predetermined adjustment has been effected with regard tothe focus detecting optical system 3. However, when the focus detectingdevice is to be assembled, it is difficult to completely effect saidpredetermined adjustment with regard to individual focus detectingoptical systems and generally as shown, for example, in FIG. 4B, thereis much possibility of an error of the function δ(F) occurring inoptical adjustment. This function δ(F) is usually a value different foreach individual focus detecting optical system and is stored inconnection with the aperture value of the photo-taking lens. The datamemory 25 for first offset is for correcting a slight error of thedefocus amount caused by insufficient mechanical adjustment after thefocus detecting device has been incorporated into a camera body and themechanical adjustment between the camera and the focus detecting devicehas been completed, and the correction amount ΔZ1 thereof also is avalue different for each individual camera. The data memory 26 forsecond offset stores therein a variable correction amount ΔZ2 to enablethe focus adjusted state to be finely adjusted in accordance with thephotographer's liking, and this correction amount can be set by anextraneous operating member, not shown.

The preset aperture value of the photo-taking lens namely, the aperturevalue to which the aperture is stopped down during photography is inputto an aperture value input means 27. Defocus amount correction means 28is provided in a microcomputer 15, and it corrects the defocus amount,which is the output of conversion means 19 by the output of the datamemory 22 for best image plane displacement and the outputs of the datamemory 25 and the data memory 26 and delivers it to display and drivingmeans 21.

The operation will now be described.

The correction amounts T(Fa, x) and δ(Fa) corresponding to the fullyopen aperture value information Fa input to the fully open aperturevalue input means 20 are read out from the correction data memory 11 foramount of image displacement and the data memory 23 for errorcorrection, respectively. The correction amount U(x) is read from thedata memory 23 for adjustment error correction. The image displacementcorrection means 18 corrects the image displacement amount Z(x) from theimage displacement operation means by the correction amounts T(Fa, x),δ(x) and U(x) and calculates the final image displacement amount ZT. Thecalculation of this final image displacement amount ZT is carried out bythe various operations described with respect to FIG. 5. The defocusamount conversion means 19 converts the final image displacement amountZT into a defocus amount by the use of the conversion factor 1/θ readout from the conversion factor memory 12 and another conversion factor Kin accordance with the fully open aperture value Fa. The defocus amountcorrection means 28 calculates the best image plane displacement amountΔβ between the best image plane at the aperture value duringphoto-taking and the best image plane at the aperture value during focusdetection by the use of the output β(F) of the data memory 22 for bestimage plane displacement, corrects the output of the conversion means 19by the best image plane displacement amount Δβ, the output ΔZ1 of thedata memory 25 for first offset and the output ΔZ2 of the data memory 26for second offset, and puts out the final defocus amount P.

Where the position displacement amount corresponding to a locationbetween the relative position of the first photoelectric element arrayand the optical image thereon and the relative position of the secondphotoelectric element array and the optical image thereon is remarkablygreat, it is sometimes difficult to correct the image displacementamount by the correction data at high accuracy. Therefore, it isdesirable to vary the pitch of the photoelectric elements of the firstand second photoelectric element arrays in accordance with the locationsthereof, to thereby pre-correct said position displacement amount to acertain degree and store the remaining position displacement amount asthe correction data.

When the position of the exit pupil of the photo-taking lens and theposition of the set pupil of the focus detecting optical system are notcoincident with each other in the direction of the optic axis, thevignetting of the focus detecting light beam imparts another adverseeffect to focus detection. This adverse effect will hereinafter bedescribed.

Referring to FIG. 13A, a plurality of small lenses 31, 32 and 33 arelinearly arranged on the back of a field lens 30 disposed rearwardly ofa photo-taking lens 1. The small lenses 31 and 33 are positioned at theopposite ends of the small lens row, and the small lens 32 is positionedat the center of the small lens row. The small lens row is determined soas to be substantially coincident with the focal plane of thephoto-taking lens 1. Pairs of photoelectric elements 34a and 34b, 35aand 35b, and 36a and 36b are disposed immediately behind the smalllenses 31, 32 and 33, respectively, and of these pairs of photoelectricelements, the lower photoelectric elements 34a, 35a and 36a togetherconstitute a first photoelectric element array and the upperphotoelectric elements 34b, 35b and 36b together constitute a secondphotoelectric element array. The first and second photoelectric elementarrays correspond to the pair of photoelectric element arrays 7A and 7B,respectively, of FIG. 1. By the small lens row and the field lens, thephotoelectric elements 34a, 35a and 36a of the first array are imaged soas to be overlapped on an area 110A and likewise, the photoelectricelements 34b, 35b and 36b of the second array are imaged so as to beoverlapped on an area 110B. These areas 110A and 110B are the set pupilsof the focus detecting optical system and correspond to the set pupils100A and 100B, respectively, of FIG. 1.

If the positions of the set pupils in the direction of the optic axisand the position of the exit pupil of the photo-taking lens in the samedirection are coincident with each other as in the case of FIG. 1, evenif the focus detecting light beam is vignetted by the photo-taking lens,this vignetting equally acts on all of the photoelectric elements andtherefore, the intensities of illumination of the photoelectric elementsare only reduced uniformly. Accordingly, this reduction in intensity ofillumination does not much adversely affect focus detection. However,where the positions 110A and 110B of the set pupils and the position 111of the exit pupil of the photo-taking lens are not coincident with eachother as shown in FIG. 13A, if the focus detecting light beam isvignetted by the photo-taking lens 1, the intensities of illumination onthe photoelectric elements will be reduced differently in accordancewith the locations of the photoelectric element arrays as willhereinafter be described in detail, and this will result in a greatreduction in focus detection accuracy. More particularly, when the exitpupil 111 is more distant from the small lens row 31, 32, 33 than theset pupils 110A and 110B, as shown in FIG. 13A, the focus detectinglight beam 112 is distributed uniformly on the central pair ofphotoelectric elements 35a, 35b as shown in FIG. 14A, but is distributednon-uniformly on the other pairs of photoelectric elements. If thepositional relation between the exit pupil 111 and the set pupils 110A,110B is opposite to that shown in FIG. 13A, the relation between thefocus detecting light beam and the pairs of photoelectric elements willbe such as shown in FIG. 14B. In FIGS. 14A and 14B, the hatching and thedistribution of numerous dots indicate the focus detecting light beamincident on the photoelectric elements 34b, 35b, 36b and the light beamincident on the elements 34a, 35a, 36a, respectively. The photoelectricoutput pattern of the first array 34a, 35a, 36a of FIG. 14A and thephotoelectric output pattern of the second array 34b, 35b, 36b areinclined in opposite directions as indicated by solid line Va(x) andbroken line Vb(x), respectively, in FIG. 15A when the brightness of theobject to be photographed is uniform. Assuming that the brightnessdistribution of any object to be photographed is the pattern indicatedby solid line in FIG. 15B, the relative displacement amount of the twophotoelectric output patterns is obtained, for example, by theoperations of the aforementioned equations (1)-(5) on the basis of thephotoelectric output of the first array and the photoelectric output ofthe second array and, when the two photoelectric output patterns aredisplaced relative to each other by this displacement amount, if thereis no vignetting of the focus detecting light beam, the photoelectricoutput pattern Fao(x) of the first array and the photoelectric outputpattern Fbo(x) of the second array will be substantially completelycoincident with each other as shown in FIG. 15B. By the vignettingcharacteristics Va(x) and Vb(x) of FIG. 15A, the photoelectric outputpatterns Fao(x) and Fbo(x) of FIG. 15B become the pattern Fa(x)indicated by solid line in FIG. 15C and the pattern Fb(x) indicated bybroken line in FIG. 15C, respectively, and the two curves becomeincoincident with each other and as a result, it becomes very difficultto detect the displacement amount of the object images on the first andsecond arrays. In FIGS. 15A, 15B and 15C, the series of photoelectricoutputs of the photoelectric element arrays are dispersive andtherefore, the curves Va(x), Vb(x), Fao(x), Fbo(x), Fa(x) and Fb(x)should be dispersively plotted, but for simplicity, they are indicatedby continuous curves.

An embodiment of the present invention which eliminates the focusdetection error resulting from such vignetting will now be described byreference to FIG. 16.

In FIG. 16, a photoelectric converting unit 40 for photoelectricallyconverting the object image by the focus detecting optical system asshown in FIGS. 1 or 13 has two pairs of photoelectric element arrays41A, 41B and 42A, 42B. One pair of photoelectric element arrays 41A, 41Band the other pair of photoelectric element arrays 42A, 42B are suchthat the size of the light-receiving portion of each photoelectricelement of the former pair is larger than the size of thelight-receiving portion of each photoelectric element of the latter pairand the set pupil positions of the two pairs are identical, but the sizeof the set pupil of the former pair is larger than the size of the setpupil of the latter pair. Accordingly, the pair of arrays 42A, 42Bdecrease in quantity of light received as compared with the pair ofarrays 41A, 41B, but the pair of arrays 42A, 42B less suffers thevignetting by the photo-taking lens than the pair 41A, 41B. That is,when a photo-taking lens of certain aperture value is used, there mayoccur a situation that the focus detecting light beam entering the pairof arrays 41A, 41B is vignetted, but the focus detecting light beamentering the pair of arrays 42A, 42B is not vignetted.

Switch means 43 alternatively selects the outputs of the photoelectricelement array 41A or 42A and likewise, switch means 44 alternativelyselects the output of the photoelectric element array 41B or 42B, andthese two switch means 43 and 44 are interlocked together so that thepair of arrays 41A, 41B are selected together and the pair of arrays42A, 42B are selected together. Signal processing means 45 effectsprocessing such as amplification or filtering of the signals from theswitch means 43 and 44. Focus detection means 46 calculates the imagedisplacement amount from the outputs of the pairs of arrays passedthrough the signal processing means 45, converts it into a defocusamount and puts out the same. Vignetting detection means 47 which willlater be described in detail receives the output signals of the pairs ofarrays from the signal processing means 45 and detects the degree ofvignetting of the focus detecting light beam. Mode change-over means 48compares the output of the vignetting detection means 47 with apredetermined value and, when the degree of vignetting is small, itproduces a normal mode signal and, when the degree of vignetting isgreat, it produces a vignetting mode signal. Driving means 49 drives thephoto-taking lens to its in-focus position in response to the defocussignal when the normal mode signal is produced. Warning means 50 warnsthat focus detection is impossible due to vignetting.

The operation will now be described.

Each array 41A, 41B, 42A, 42B time-serially produces a series ofphotoelectric outputs representative of the illumination intensitypattern of the object image on itself at a predetermined time interval.Switch means 43 and 44 normally select the pair of photoelectric elementarrays 41A, 41B comprising photoelectric elements having largelight-receiving areas. Focus detection means 46 calculates the imagedisplacement amount from the output signals of the pair of arrays 41A,41B passed through the signal processing means 45, and converts it intoa defocus amount. The vignetting detection means 47 detects the degreeof vignetting on the basis of the outputs of the pair of arrays 41A,41B. The mode change-over means 48 produces a normal mode signal when itjudges that the output of the means 47 is small. At this time, thedriving means 49 drives the photo-taking lens to its in-focus positionin response to the defocus signal of the means 46. The above-describedoperation is repeated during the time that the normal mode signal isproduced.

The mode change-over means 48 produces a vignetting mode signal when itjudges that the output of the means 47 is great, and this vignettingmode signal decelerates or stops the driving of the photo-taking lens bythe driving means 49 and causes the switch means 43 and 44 to select thepair of arrays 42A, 42B. Thus, the focus detection means 46 detects theimage displacement amount on the basis of the output signals of the pairof arrays 42A, 42B, and converts it into a defocus amount by aconversion factor selected in accordance with the vignetting modesignal. Also, the vignetting detection means 47 detects the degree ofvignetting. The mode change-over means 48 produces a normal mode signalwhen it judges that the degree of vignetting is small, and thus thedriving means 49 drives the photo-taking lens in response to the thendefocus signal. The above-described operation is repeated during thetime that this normal mode signal is produced.

A vignetting mode signal is produced when the mode change-over 48 judgesthat the degree of vignetting is great in spite of the pair of arrays42A, 42B being selected, and thereby the driving of the photo-takinglens is decelerated or stopped. The warning means 50 warns and displaysthat focus detection is impossible by the then photo-taking lens, inresponse to the vignetting mode signal when the pair of arrays 42A, 42Bare selected.

Specific examples of the vignetting detection means and mode change-overmeans will now be described.

The value "Cext" of equation (4') represents the area of the regionindicated by hatching in FIG. 15C, and this properly reflects the degreeof vignetting and accordingly becomes great in accordance with thedegree of vignetting. Thus, as shown in FIG. 17A, the operation of theafore-mentioned equations (1)-(4') is effected by correlation operationmeans 51 which receives the output signals of the pair of arrays 41A,41B or 42A, 42B, and the value "Cext" is found. This value is comparedwith the reference value output of reference value producing means 52 bycomparing means 53. The comparing means 53 produces a mode change-oversignal when the former is greater than the latter, and produces a normalmode signal when the former is smaller than the latter. The correlationoperation means 51 constitutes the vignetting detection means 47, andthe reference value producing means 52 and the comparing means 53together constitute the mode change-over means 48. Also, since thecorrelation amount C(L) differs greatly depending on the object to bephotographed, the value "Cext" may benormalized. For example, Cext/Cmaxobtained by dividing "Cext" by a maximum correlation amount Cmax or theCext of equation (4) may be used instead of Cext.

If the focus detection means 46 of FIG. 16 is one which effects theoperation of equations (1)-(5) and calculates the image displacementamount Lm, the result "Cext" or Cext of the operation of this focusdetection means 46 can be intactly utilized for the detection ofvignetting and therefore, the vignetting detection means 47 can beeliminated.

Another specific example of the construction of the vignetting detectionmeans and mode changeover means will now be described.

As shown in FIG. 15C, the photoelectric output pattern Fa(x) of thefirst array and the photoelectric output pattern Fb(x) of the secondarray, even if made coincident with each other in the direction x, arenot coincident with each other in magnitude due to the vignettingcharacteristics Va(x) and Vb(x) of FIG. 15A, and on the right side withthe center of the pattern, i.e., x=0, as the boundary, one pattern is.larger than the other pattern and, on the left side, one pattern issmaller than the other pattern, and a difference as indicated byhatching occurs between the sizes of the two patterns Fa(x) and Fb(x).The magnitude of the area of this hatching portion depends on the degreeof vignetting and therefore, by finding this area, the degree ofvignetting can be detected. FIG. 17B shows an example in which this areais found, and the correlation operation means 51 finds the relativedisplacement amount Lm of the two photoelectric output patterns on thebasis of the photoelectric outputs of the first and second arrays.Displacing means 54 displaces the photoelectric output pattern Fa(x) ofthe first array relative to the photoelectric output pattern Fb(x) ofthe second array by said displacement amount Lm in the direction x andmakes the two patterns Fa(x) and Fb(x) coincident with each other in thedirection x, as shown in FIG. 15C. Right side integration means 55 andleft side integration means 56 receive as inputs the photoelectricoutputs of the first and second arrays in which the relativedisplacement amount has thus become zero, and the right side and leftside integration means 55 and 56 integrate Fa(x)-Fb(x) with respect to acertain area on the right side of FIG. 15C and a certain area on theleft side of FIG. 15C, respectively. As described above, the patternsFa(x) and Fb(x) are opposite in magnitude on the right side and the leftside and therefore, the outputs of the right side and left sideintegration means are opposite in sign.

Subtracting means 57 finds the difference between the outputs of the twointegration means 55 and 56, and absolute value means 58 finds theabsolute value of the output of the means 57. Accordingly, the output ofthe absolute value means 58 represents the total area of a part of theright side hatching portion of FIG. 15C and a part of the left sidehatching portion of FIG. 15C and thus, the degree of vignetting.Comparing means 53 compares this absolute value output with a referencevalue and makes a normal mode signal or a vignetting mode signal.

The integration ranges of the integration means 55 and 56 may be theentire right side range and the entire left side range, respectively, ofthe center x=0 of FIG. 15C, or may be only the small areas greatlyspaced apart to the right and left from the center at which theinfluence of vignetting is great.

Another specific example of the construction of the vignetting detectionmeans and mode change-over means will now be described.

Functions Va(x) and Vb(x) representing the vignetting shown in FIG. 15Acan be expressed by the following equations:

    Va(x)=1+βx

    Vb(x)=1-βx

The factor β showing the slope of Va(x) and Vb(x) corresponds to thedegree of vignetting. This factor β can be calculated as follows.

The pair of photoelectric output patterns Fa(x) and Fb(x) of FIG. 15Ccan be expressed as follows from the above-mentioned vignettingfunctions Va(x) and Vb(x) and functions Fao(x) and Fbo(x) representativeof the object brightness.

    Fa(x)=Fao(x)×(1+βx)                             (6)

    Fb(x)=Fao(x)×(1-βx)                             (7)

    Also, Fao(x)=Fbo(x)                                        (8)

From equations (6)-(8), β is expressed as: ##EQU5##

In FIG. 17C, correlation operation means 51 and displacing means 54 arethe same as those of FIG. 17B. Slope detection means 59 receives fromthe displacing means 54 a pair of photoelectric outputs Fa(x) and Fb(x)of which the relative displacement amount has become zero as shown inFIG. 15C, and effects the operation of equation (9) by introducing aparticular value into x, and obtains the slope β. Preferably, theparticular value for "x" is the value at a location relatively spacedapart from the origin (x=0) in FIG. 15A, and a plurality of such valuesare chosen and the slope β is found for each of those values, and theaverage value of them is finally used as β. Absolute value means 58 andcomparing means 53 are the same as those of FIG. 17B.

Reference is now had to FIGS. 18 and 20 to describe two furtherembodiments in which the reduction in focus detection accuracy resultingfrom vignetting described in connection with Figures 15A-15C isprevented.

In FIG. 18, a photoelectric unit 60 has first and second photoelectricelement arrays 60A and 60B. These arrays 60A and 60B are the same as thefirst and second arrays 7A and 7B of FIG. 1 or the first arrays 34a,35a, 36a and the second arrays 34b, 35b, 36b of FIG. 13. Switch means 61receives a series of output signals a1, . . . , aN of the first arrayand a series of output signals b1, . . . , bN of the second array fromthe photoelectric unit 60 and alternatively sends them to first filtermeans 62 and second filter means 63. The first filter means 62 has anMTF (modulation transfer function) characteristic which has pass bandfrom DC component to frequency fn/2, as shown in FIG. 19A. In FIG. 19A,the abscissa represents spatial frequency and fn is 1/(2 d), where d isthe pitch of the photoelectric elements of the photoelectric elementarray. Accordingly, this filter means 62 passes therethrough thecomponents from the DC component to the vicinity of about fn/ 2, of thespatial frequency component of the optical image on the array which isincluded in the series of output signals. The second filter means 63 hasan MTF characteristic that as shown in FIG. 19B, it passes therethroughthe spatial frequency component in the vicinity of fn/4 and removes theDC component, the component in the vicinity thereof and the component offn/2 or more. Focus detection means 64 is the same as the means 46 ofFIG. 16, and it detects the relative displacement amount of the opticalimages on the arrays 60A and 60B, converts it into a defocus amount andputs out the same. Drive and display means 65 drives the photo-takinglens in response to the defocus signal and displays the focus adjustedstate of the lens.

Vignetting detection means 66 is the same as the means 47 of FIG. 16,and mode change-over means 7 receives the output of the means 66 andsends a normal mode signal or a vignetting mode signal to the switchmeans 61. The switch means 61 sends the output signal of thephotoelectric unit 60 to the first filter means 62 in response to thenormal mode signal, and to the second filter means 63 in response to thevignetting mode signal.

The operation will hereinafter be described.

Let it be assumed that the switch means 61 selects the first filtermeans 62. The output signals from the pair of arrays 60A and 60B arefiltered by the first filter means 62, and then enter the focusdetection means 64. The means 64 calculates a defocus amount therefrom,and the means 65 effects the driving of the photo-taking lens anddisplay in accordance with the defocus amount. The vignetting detectionmeans 66 detects the degree of vignetting of the focus detecting lightbeam from said output signals, and the change-over means 67 produces anormal mode signal in accordance with the output of the means 66 whenthe degree of vignetting is small, and produces a vignetting mode signalwhen the degree of vignetting is great. The switch means 61 maintainsthe selection of the first filter means 62 in accordance with the normalmode signal, and selects the second filter means 63 in accordance withthe vignetting mode signal. By the selection of the second filter means63, the output signal from the photoelectric unit 60 is filtered inaccordance with the MTF characteristic of FIG. 19B. Generally, thevignetting of the focus detecting light beam greatly affects the lowfrequency component including the DC component of the above-mentionedoutput signals, and its influence upon higher frequency components isvery small. Accordingly, when the low frequency component is removedfrom the output signals by the second filter means 63, the filteredoutput signals are scarcely affected by vignetting. The focus detectionmeans 64 calculates a defocus amount on the basis of the output of thesecond filter means 63, and the means 65 effects said driving anddisplay in response to this defocus amount.

In FIG. 20, switch means 61 selects signal processing means 70 inresponse to the normal mode signal, and selects the series connectingmember of logarithmic means 71 and second difference means 72 inresponse to the vignetting mode signal. The signal processing means 70amplifies or suitably filters the input signal. The combination of thelogarithmic means 71 and the second difference means 72 has the functionof very effectively removing the components which are included in aseries of output signals a1, . . . , aN of a first array 60A and aseries of output signals b1, . . . , bN of a second array 60B and whichhave been affected by vignetting. The logarithmic means 71 converts thetime-serially sent output signals a1, . . . , aN and b1, . . . , bN intologarithms log a1, . . . , log aN and log b1, . . . , log bN. Theselogarithmic outputs log a1, . . . , log aN and log b1, . . . , log bNare defined as A1, . . . , AN and B1, . . . , BN, respectively. Thesecond difference means 72 finds the second differences of thelogarithmic outputs A1, . . . , AN and B1, . . . , BN time-serially sentfrom the logarithmic means 71. The second differences mean, for example,(A1-2A2+A3), (A2-2A3+A4), . . . when the series of original signals areA1, A2, A3, A4, . . . . These second differences correspond to the firstdifferences of new original signals when the first difference signals(A2-A1), (A3-A2), (A4-A3), . . . of the original signals A1, A2, A3, A4,. . . are found and when these first difference signals (A2-A1),(A3-A2), . . . are the new original signals. The differences are notlimited to the differences (A2-A1), (A3-A2), . . . between adjacent dataas described above, but may also be the differences (A3-A1), (A4-A2), .. . of every other data or the differences (A4-A1), (A5-A2), . . . ofevery two data.

The action of the second difference means is to filter the input signalin accordance with a predetermined MTF characteristic The MTFcharacteristic of FIG. 19B can be obtained by second difference meanswhich finds the second differences of every other data, i.e., (A1-2A3+A5), (A2-2A4+A6).

FIG. 21 shows an example of the second difference means 72. Thetime-serial signals A1, AN and B1, . . . , BN from the logarithmic means71 are successively sent to series-connected delay means 73, 74 and 75.Multiplication means 76, 77 and 78 have coefficients -1, 2 and -1,respectively, and multiply the contents of the delay means 73, 74 and75, for example, A3, A2 and A1, by their coefficients -1, 2 and -1,respectively. Addition means 79 adds together the outputs of themultiplication means 76-78. Accordingly, the addition means 79 puts outsecond difference outputs (-A1+2A2-A3), (-A2+2A3-A4), . . . insuccession as signals A1, A2, A3, A4 are successively input from thelogarithmic means 71 to the delay means.

It will hereinafter be illustrated that the logarithmic means 71 and thesecond difference means 72 reduce the influence of vignetting upon theoutput signal.

The photoelectric output patterns Fa(x) and Fb(x) of FIG. 15C areexpressed as follows by substituting F(x) for Fao(x) of equation (6) andFbo(x) of equation (7):

Fa(x)=F(x) x (1+βx)

Fb(x)=F(x) x (1-βx)

After the logarithms of Fa(x) and Fb(x) have been obtained, the seconddifference outputs Δ² Fa(x) and Δ² Fb(x) at two points spaced apart by apredetermined amount d are as follows: ##EQU6##

The difference D(x) between these second difference outputs Δ² Fa(x) andΔ² Fb(x) is: ##EQU7##

An attempt is made to find this difference D(x) from a specific exampleof numerical values. When the length of each photoelectric element arrayis 6 mm and d=0.6 mm and vignetting functions Va(x) and Vb(x) at theopposite ends x±3 mm of the array whereat the influence of vignetting isgreatest are greater or smaller by 30% than the center of the array, thedifference D(x) at the opposite ends of the array is about -0.005.

This value -0.005 is appreciably smaller than in a case where no meansis provided for reducing the influence of vignetting, and further, isgreatly smaller than the similar value D(3)=-0.13 by a combination ofthe logarithmic means and the first difference means which can reducethe influence of vignetting, and it can be seen from this that thecombination of the logarithmic means and the second difference means inaccordance with the present invention is very effective for the removalof the influence of vignetting.

The action of the arrangement of FIG. 20 is similar to that of thearrangement of FIG. 18, and the defocus amount is calculated on thebasis of the output signals a1, . . . , aN and b1, . . . , bN processedby the signal processing means 70 when the normal mode signal isproduced, and is calculated on the basis of the output signals processedby the means 71 and 72 when the vignetting mode signal is produced.

In FIGS. 18 and 20, the vignetting detection means 66 receives as inputthe output produced by the focus detection means 64, for example, theimage displacement amount Lm, as indicated by dotted line, and candetect vignetting on the basis thereof. In this case, whether theinfluence of vignetting has been sufficiently removed in the state inwhich the second filter 63 and the logarithmic means 71 are selected canbe detected, and if the influence of vignetting has not been removed, awarning can be produced by the use of means similar to the warning means50 of FIG. 16.

In the foregoing, change-over of the filter means, etc. has beeneffected when the degree of vignetting is great, but instead thereof orin addition thereto, the area of the series of output signals used inthe focus detection means may be changed More particularly, as shown inFIG. 15A, the influence of vignetting is small in the vicinity of thecenter of the photoelectric element array and becomes greater toward theopposite ends thereof and therefore, the output signals corresponding tothe vicinity of the center of the array is less affected by vignetting.Thus, during the normal mode, the focus detection means is caused tocalculate the defocus amount by the use of all of the series of outputsignals and, during the vignetting mode, the focus detection means iscaused to calculate the defocus amount on the basis of only the outputsignals of said series of output signals in the vicinity of the center.

Logarithmic conversion means used for the logarithmic means 71 of FIG.20 usually converts an input X into an output Y by the logarithmicconversion characteristic indicated by dotted line l₁ in FIG. 22.However, in the conversion means having such a conversion characteristicl₁, the output Y will assume a great negative value if the input Xbecomes smaller, and it will exceed the dynamic range of the subsequentcircuit (in FIG. 20, the second difference circuit 72) which receivesthis output Y, thereby reducing the focus detection accuracy.

To avoid such an inconvenience, use may be made of the conversioncharacteristic indicated by solid line l₂ or l₃ in FIG. 22. Thisconversion characteristic l₂ comprises a straight line portion l_(2a) inthe area wherein the input X is small and a logarithmic conversion curveportion l_(2b) in the area wherein the input X is great. The straightline portion l_(2a) passes through the origin O and smoothly connects tothe logarithmic conversion curve portion l_(2b). This characteristic l₂may be mathematically expressed as follows. The relation in thelogarithmic conversion curve portion l_(2b) is: ##EQU8## (X≧E·e, where eis the value of the bottom of the logarithm)

The relation in the straight line portion l_(2a) is: ##EQU9## (X<E·e)

The characteristic l₂ is such that in the equations above, A=200, E=18.5and E·e≈50.

The series of signals input to such logarithmic conversion means arepredetermined so that the average value thereof is substantially themedian of the input range 0<X<A (=200) of the conversion means, namely,A/2.

In the above equations, the parameter E is an amount related to thedegree of curvature of LOG curve. Where E is too great, the curve l₂shown in FIG. 22 becomes approximate to a straight line as a whole andcannot sufficiently display the effect by the LOG characteristic, andwhere E is too small, the curvature of the curve l₂ becomes sharp andthe output Y is extremely compressed relative to the input X and thefluctuation range of the output Y becomes too narrow.

With the foregoing fact taken into account, where equation (6) isapplied, it is necessary that the relation between the parameter E and Abe ##EQU10## and it has been found from the result of the experimentcarried out with respect to various objects to be photographed that agood result is obtained for ##EQU11## and a very good result is obtainedfor ##EQU12##

The logarithmic conversion curve l₃ of FIG. 22 has been obtained byparallel-moving the curve l₁ in the direction of X-axis so that whenX=0, Y=0, and may be mathematically expressed as follows: ##EQU13##

The curve l₃ refers to a case where A=200 and B=200 and E=18.5.

In the above equation, the parameter E/A may preferably be ##EQU14##

The conditions for preventing the preferable logarithmic conversioncharacteristics as indicated, for example, by the characteristics l₂ andl₃ are as follows:

○2 The input X and the output Y of the logarithmic conversion meansshould be in the relation of monotonous increase or monotonous decrease.If not so, there will be provided a result similar to the result thatthe optical image on the photoelectric element array has beenphotoelectrically converted in locally distorted manner.

○3 When the ranges of the input X and the output Y are X₁ ≦X≦X₂ and Y₁≦Y≦Y₂, in order that the output Y may not be excessively compressed in apart of the input range, it is desirable that ##EQU15## where dY/dX is adifferential coefficient in ##EQU16##

FIGS. 23A and 23B show specific examples of the circuit of conversionmeans having logarithmic conversion characteristics l₂ and l₃. Avoltage-current converting circuit comprises resistors R1, R2,transistors T1, T2, T3, T4 and a constant current source Q1, andconverts the difference between an input voltage Vx and a referencevoltage VY into a current i proportional thereto.

The diodes D1-D5, D11-D16, transistor T5, constant current source Q2 andoperational amplifier P1 of FIG. 23A convert the input current i into anoutput voltage Vy in accordance with the characteristic l₂. The diodesD1-D4, D11-D14, constant current sources Q2, Q3 and operationalamplifier Pl of Figure 3B convert the input current i into an outputvoltage Vy in accordance with the characteristic l₃.

FIG. 24 shows an example in which a plurality of logarithmic conversionmeans having different characteristics are prepared and the logarithmicconversion means are selected in accordance with the magnitudes of theseries of output signals from the photoelectric element array.

In FIG. 24, minimum value detection means 80 detects the minimum valuesof the series of output signals a1, . . . , aN and b1, . . . , bN fromthe switch means 61 (FIG. 20). It is to be understood that themagnitudes of these output signals are within a predetermined range, forexample, the range of 0-200 and the average value thereof isapproximately 100, by the control of the charge accumulating time of aphotoelectric element array such as a CCD type image sensor. The seriesof output signals are delayed by a delay circuit 81 until the minimumvalue detection circuit 80 detects the minimum values, whereafter theyare input to selection means 82. The selection means 82 alternativelyselects logarithmic conversion means 83, 84, 85 in accordance with theoutput of the minimum value detection means 80. The means 83, 84 and 85have conversion characteristics l₄, l₅ and l₆, respectively, shown inFIG. 25. The characteristic l₄ converts an input in the range of 50-200into an output in the range of 0-200, the characteristic l₅ converts aninput in the range of 20-200 into an output in the range of 0-200, andthe characteristic l₆ converts an input in the range of 12.5-200 into anoutput in the range of 0-200 and an input in the range of 0-12.5 into anoutput of 0. When the detection means 80 detects that the minimum valueof the series of output signals is 50 or more, the selection means 82selects the conversion means 83 and causes said output signals to beconverted in accordance with the characteristic l₄. Likewise, when theminimum value is 25-50, the conversion means 84 is selected and, whenthe minimum vlaue is less than 25, the conversion means 85 is selected.The logarithmically converted output signals are sent to the seconddifference means 72.

In this manner, the conversion means are selected in accordance with thedistribution of the input signal and therefore, optimum logarithmicconversion can be accomplished. Such conversion means may be constitutedby circuits, or may be stored as a logarithmic conversion table in theROM within a microprocessor.

We claim:
 1. A focus detecting device for detecting a relationshipbetween a predetermined plane and an image, which is formed by animaging optical system, of an object to be photographed, said devicecomprising:a focus detection system provided with first and secondphotoelectric arrays each including a plurality of photoelectricelements arranged in one direction, a detection optical system fordirecting a first radiation beam to said first photoelectric array andfor directing a second radiation beam to said second photoelectric arrayand focus detection means, said first radiation beam passing through afirst area of an exit pupil of said imaging optical system from saidobject, said second radiation beam passing through a second area of saidexit pupil of said imaging optical system from said object, said secondarea being different from said first area, said first photoelectricarray producing a first electrical signal corresponding to an intensitydistribution of incident light on said plurality of photoelectricelements thereof, said second photoelectric array producing a secondelectrical signal corresponding to an intensity distribution of incidentlight on said plurality of photoelectric elements thereof, said focusdetection means producing a focus detection signal representative ofsaid relationship in response to said first and second electricalsignals; and vignetting detection means responsive to said first andsecond electrical signals for calculating an amount of a vignetting ofsaid first and second light beams which is caused by said imagingoptical system.
 2. A focus detecting device according to claim 1,wherein said vignetting detection means comprises comparison means forcomparing said amount with a reference value and thereafter producing acomparison signal.
 3. A focus detecting device according to claim 2,wherein said vignetting detection means comprises correlation means fordetecting a correlation between said first and second electrical signalsand producing a correlation signal indicative of said amount, and saidcomparison means compares said correlation signal with said referencevalue.
 4. A focus detecting device according to claim 2, wherein saidvignetting detection means comprises means responsive to said first andsecond electrical signals for calculating a relative displacement amountbetween a pattern of the intensity distribution represented by saidfirst electrical signal and a Pattern of the intensity distributionrepresented by said second electrical signal and for producing adisplacement signal indicative of said relative displacement amount,means responsive to said displacement signal for detecting a differencebetween at least one part of said first electrical signal and at leastone part of said second electrical signal, each of which correspondswith the same area of said object and for producing an output signalcorresponding said amount.
 5. A focus detecting device according toclaim 2, wherein said vignetting detection means comprises meansresponsive to said first and second electrical signals for calculating avalue of a slope of a primary function which indicates a tendency of theintensity distribution of one of said first and second electricalsignals and for producing an output signal indicative of the value ofsaid slope, and wherein said comparison means compares said outputsignal with said reference value.
 6. A focus detecting device accordingto claim 2, wherein said focus detection means comprising first filtermeans for filtering one part of a component of said first and secondelectrical signals and outputting a first output signal corresponding tosaid one part filtered by said first filter means, second filter meansfor filtering another part of the component of said first and secondelectrical signals and outputting a second output signal correspondingto said another Part filtered by said second filter means, and selectionmeans responsive to said comparison signal for selecting one of saidfirst and second output signals, and wherein said focus detection meansproduces said focus detection signal in response to said one of saidfirst and second output signals selected by said selection means.
 7. Afocus detecting device according to claim 2, which further comprisesdriving means for driving said imaging optical system in accordance withsaid focus detection signal, and control means responsive to saidcomparison signal for controlling said driving means.
 8. A focusdetecting device according to claim 7, wherein said control means causessaid driving means to stop said driving means or suppresses said drivingof said driving means.
 9. A focus detecting device according to claim 2,wherein said focus detection means comprises Processing means forprocessing said first and second electrical signals so as to remove onepart of components of each of said first and second electrical signals,said one part of components having been subjected to influence of saidvignetting, and wherein said focus detection means produces said focusdetection signal on the basis of said first and second electricalsignals processed by said processing means in response to saidcomparison signal.
 10. A focus detecting device according to claim 2,wherein said focus detection means comprising first processing means forfiltering one part of a component of said first and second electricalsignals and producing an output, second processing means for convertingsaid first and second output signals into an output associated withlogarithms thereof, and selection means responsive to said comparisonsignal for applying said first and second electrical signals to one ofsaid first and second processing means, and wherein said focus detectionmeans produces said focus detection signal in response to one of saidoutputs of said first and second processing means.
 11. A focus detectingdevice according to claim 2, wherein said focus detection meanscomprises a plurality of processing means, and selection meansresponsive to said comparison signal for applying said first and secondelectrical signals to one of said plurality of processing means, whereineach of said Plurality of processing means has a different logarithmicfunction and converts said first and second electrical signals into aprocessed signal one basis of said logarithmic function, and whereinsaid focus detection means produces said focus detection signal inresponse to a processed signal of one of said plurality of processingmeans.
 12. A focus detecting device according to claim 2, wherein saidfocus detection system comprises third and fourth photoelectric arrayseach including a plurality of photoelectric elements arranged in onedirection and selection means, said third photoelectric array producinga third electrical signal corresponding to the intensity distribution ofincident light on said plurality of photoelectric elements thereof, saidfourth photoelectric array producing a fourth electrical signalcorresponding to the intensity distribution of incident light on saidplurality of photoelectric elements thereof, and selection means thatinputs said second and third electrical signals instead of said firstand second electrical signals to said focus detection means in responseto said comparison signal, and wherein said detection optical systemdirects a part of said first radiation beam to said third photoelectricarray and directs a part of second radiation beam to said fourthphotoelectric array.
 13. A focus detecting device according to claim 1,wherein said vignetting detection means comprises means responsive tosaid first and second electrical signals for calculating a relativedisplacement amount between a pattern of the intensity distributionrepresented by said first electrical signal and a pattern of theintensity distribution represented by said second electrical signal andfor producing a displacement signal indicative of said relativedisplacement amount, displacement means responsive to said displacementsignal for imparting a displacement between said first and secondelectrical signals, first integration means responsive to saiddisplacement means for calculating an integration value of a differencebetween one part of said first electrical signal and one part of saidsecond electrical signal, second integration means responsive to saiddisplacement means for calculating an integration value of a differencebetween another part of said first electrical signal and another part ofsaid second electrical signal, and means for calculating the amount ofsaid vignetting on the basis of the integration values calculated bysaid first and second integration means.
 14. A focus detecting devicefor detecting a relationship between a predetermined plane and an imageformed by an imaging optical system of an object to be photographed,said device comprising:a focus detection system provided with first andsecond photoelectric arrays each including a plurality of photoelectricelements arranged in one direction, a detection optical system fordirecting a first radiation beam to said first photoelectric array andfor directing a second radiation beam to said second photoelectric arrayand focus detection means, said first radiation beam passing through afirst area of an exit pupil of said imaging optical system from saidobject, said sec6nd radiation beam passing through a second area of saidexit pupil of said imaging optical system from said object, said secondarea being different from said first area, said first photoelectricarray producing a first electrical signal corresponding to an intensitydistribution of incident light on said plurality of photoelectricelements thereof, said second photoelectric array producing a secondelectrical signal corresponding to an intensity distribution of incidentlight on said plurality of photoelectric elements thereof, said focusdetection means producing a focus detection signal representative ofsaid relationship in response to said first and second electricalsignals; vignetting detection means responsive to said first and secondelectrical signals for detecting that said imaging optical system causesvignetting of said first and second light beams, said vignettingdetection means being provided with means for calculating an amount ofsaid vignetting in accordance with said first and second electricalsignals and comparison means for comparing said amount with a referencevalue and thereafter producing a comparison signal; and control meansresponsive to said comparison signal for controlling said focusdetection system.
 15. A focus detecting device according to claim 14,wherein said focus detection means comprises processing means forprocessing said first and second electrical signals, and wherein saidcontrol means controls said processing means in response to saidcomparison signal.